Problem

Source: Romania National Olympiad 2016, grade ix, p.2

Tags: inequalities, n-variable inequality



Let be a natural number $ n\ge 2 $ and $ n $ positive real numbers $ a_1,a_n,\ldots ,a_n $ that satisfy the inequalities $$ \sum_{j=1}^i a_j\le a_{i+1} ,\quad \forall i\in\{ 1,2,\ldots ,n-1 \} . $$Prove that $$ \sum_{k=1}^{n-1} \frac{a_k}{a_{k+1}}\le n/2 . $$